1. Field of the Invention
The present invention relates to a picture-in-picture system using quincunx sampling to improve horizontal resolution.
2. Description of the Related Art Including Information Disclosed Under C.F.R. 1.97 and 1.98
Present image display systems include the ability to display a small auxiliary image in addition to a larger main image. This smaller image may be displayed within the boundaries of the larger main picture, in which case, such a system is termed a picture-in-picture (PIP) system, or the smaller image may be located outside (e.g. to the left or right side of the main image, in which case the system is termed a picture-outside-picture (POP) system. The main and auxiliary images may be derived from the same image source, such as a freeze frame PIP image of the main image, or may be derived from an independent source, such as a system in which one tuner tunes one video signal which is displayed as the main image, and a second tuner tunes a second video signal, independent of the first tuner, which is displayed as the inset image.
A PIP or POP system operates by storing compressed image data representing the auxiliary image as it occurs in the auxiliary video signal, and then substituting this compressed image data for the main image signal at the portion of the main image which is designated to display the auxiliary image. The system must supply an amount of memory sufficient to store the auxiliary image data from the time it occurs in its video signal to the time it is displayed in the main image. Known systems provide sufficient memory to hold either a frame or a field of auxiliary video data. Because memory is relatively expensive, it is desirable to minimize the amount of memory required. To decrease the amount of memory required, known PIP and POP systems subsample the auxiliary video signal, and store only a single field of subsampled auxiliary video data. A display method, complementary to the subsampling method, is used to display the PIP or POP image.
Known subsampling techniques, however, consist of straightforward `take one sample, discard N samples` repeated for each line in the auxiliary video signal. This form of subsampling takes samples in a rectangular pattern, and is termed rectangular subsampling below. This undesirably decreases the horizontal resolution of the PIP or POP image, which, in turn, decreases the perceived quality of the displayed PIP or POP image. A subsampling method which can increase the horizontal resolution of a PIP or POP image, without increasing the amount of memory necessary to store the PIP or POP image data for later display with the main image is desirable.
Quincunx subsampling is one method to increase the perceived horizontal resolution of the subsampled auxiliary image without increasing the amount of memory required to temporarily store the subsampled auxiliary image data. A quincuncial pattern is defined, in terms of spatial sampling, as four samples arranged in a square or rectangle and one sample in the middle of the square or rectangle. Quincunx sampling involves subsampling one line of the auxiliary image at a first set of equally spaced horizontal locations, and subsampling the next adjacent horizontal line (which, in a standard interlaced television video signal, is in the next field) at a second set of horizontal locations halfway between the first set, and repeating this pattern. The top portion of FIG. 3, described in more detail below, illustrates this sampling technique where the "X"s represent subsampled samples, and the "+"s represent skipped samples. As can be seen, and as will be described below, the same number of samples are taken in each horizontal line, but the samples, taken all together, cover twice as many horizontal locations, thus increasing the perceived horizontal resolution of the subsampled auxiliary image.
Because spatially adjacent lines occur in temporally adjacent fields, quincunx sampling may be accomplished by subsampling lines in one field of the auxiliary image at one set of horizontal locations, and subsampling lines in the next adjacent field at a second set of horizontal locations halfway between the horizontal locations of the preceding field. This pattern is repeated. Thus, each line within a field is sampled identically to all the other lines in that same field, but the horizontal locations of subsampled samples in the lines in one field are halfway between the horizontal locations of the subsampled samples in lines in the adjacent fields.
A problem occurs in PIP or POP systems using quincunx sampling. Because the main and auxiliary video signals are not synchronized, when a field of subsampled auxiliary image samples is to be retrieved from the memory and displayed in the main image, one portion of the memory temporarily contains samples taken from a preceding auxiliary image field, while the other portion of the memory contains samples taken from the current auxiliary image field. Because these fields are 1/60 second apart (in the United States NTSC standard) a seam, termed a temporal seam, occurs at the juxtaposition of the image portions formed from the samples retrieved from these two portions of the memory. If, additionally, samples from the preceding field were taken using one sampling pattern, while samples in the current field are taken with the other sampling pattern, then when these samples are combined with the main image and displayed, there is a seam, termed a spatial seam, at the temporal seam location between the samples taken from the preceding auxiliary field, and those taken from the current auxiliary field. This seriously degrades the quality of the inset auxiliary image. It is desirable to provide for the perceived increased horizontal resolution provided by quincunx sampling without the degradation resulting from the spatial seam at the temporal seam location.
The spatial seam at the temporal seam location may be eliminated by monitoring the location of the temporal seam, and subsampling the auxiliary image using one sample pattern before the temporal seam, and using the other sample pattern after the temporal seam. In this manner, when samples representing the auxiliary inset image are retrieved from memory, the samples in the portion of the memory containing samples from the preceding field have been sampled using the same sampling pattern as those in the portion of the memory containing samples from the current field. This will eliminate the spatial seam at the temporal seam location.
One function provided by current video display devices is a freeze frame operation for the inset auxiliary image. This function is provided by suspending the writing of new samples into the field memory, while reading of samples from the field memory, and generation of an inset image, continues. Because no new data is written into the field memory, the inset image doesn't change, and appears fixed or frozen. If, however, the inset image signal is quincunx subsampled in the manner described above, to eliminate the spatial seam at the temporal seam location, then when the writing of auxiliary image subsampled samples into the field memory is suspended, one portion of the field memory will contain samples taken before the temporal seam location and sampled using one sampling pattern, while the other portion of the field memory will contain samples taken after the temporal seam location and sampled using the other sampling pattern. When the frozen inset image represented by these samples is displayed, it will have a spatial seam at the temporal seam location. It is desirable to provide quincunx subsampling for the inset auxiliary image to provide higher perceived horizontal resolution, without introducing a spatial seam at the temporal seam location, during a freeze frame operation, when a frozen auxiliary inset image is displayed.